This invention relates generally to phased array radar systems and, more particularly, to an illumination taper adjusting apparatus and method which provides scan compensation for a phased array antenna on a curved surface.
In phased array microwave radar systems, it is often required in a monopulse feed network to form two or more simultaneous beams on receive having different weightings. As an example, it may be required in a monopulse feed network to form a sum beam having Taylor weighting and a difference beam having a Bayliss weighting, along a linear array of, illustratively, sixty-four radiating elements.
The curvature of a conformal phased array antenna distorts the radiation pattern when the beam is scanned. In the prior art a paper by John Antonucci and Peter Franchi titled "A Simple Technique to Correct for Curvature Effects on Conformal Phased Arrays," Proceedings of the 1985 Antenna Applications Symposium, Rome Air Development Command, Report No. RADC-TR-85-242, Vol. 2, December 1985, describes a technique of using the sum and difference networks in combination to correct for curvature effects. A variable power divider is used to combine the power in a prescribed proportion at an arbitrary scan angle between the sum and difference channels. This recombination method partially restores the original aperture illumination for the scanned direction. The optimum amount of signal to be distributed to achieve the maximum restoration is found as a function of scan angle and curvature. However, this approach to correct conformal array curvature effects only partially corrects the illumination taper and still results in high sidelobes. In order to fully correct for conformal effects in the prior art, separate phase shifters and attenuators are placed at each radiating element, one for each beam, in order to properly correct for curvature effects. This represents a severe cost multiplier for the fabrication of curved phased array antennas.
The beamforming architecture of the prior art typically uses each column of a phased array to generate simultaneously sum and difference patterns on receive beams. Typically, these beamformers are used in planar arrays where scan compensation of the illumination is not needed or done when the beam is scanned. Similar architectures may be used to combine columns into a two dimensional array. Typically, a single T/R module with a single phase shifter and level set attenuator is used at each radiating element in the phase array. When building a curved or conformal array, it is necessary to limit the T/R module at the radiating elements to one phase shifter and attenuator, as is done with planar arrays, in order to keep array cost, size, volume and weight at reasonable levels.